Darius I, also known as Darius the Great, was the third king of the Achaemenid Empire, ruling from 522 to 486 BCE. He is best known for his administrative reforms, which helped to unify and manage the vast territories of the empire. Darius introduced a system of satrapies, or provinces, each governed by a satrap, which improved local governance and tax collection. Darius I is also famous for his construction projects, including the grand palace at Persepolis and the Royal Road, which facilitated trade and communication across the empire. His reign marked a period of expansion and stability, making him one of the most significant rulers in ancient history.